316 A TEXTBOOK OF BOTANY [Cn. VI, 8 



there is not only no known evidence, but such evidence as 

 we possess seems wholly against its occurrence, with possible 

 rare exceptions which hardly affect the general principle. All 

 evidence seems to show that while alterations in the deter- 

 miners alter the organism, the reverse is not true. 



A second solution, and the most famous, is that of Darwin, 

 who was active in his work somewhat over a half century ago. 

 He argued that a spontaneous variation of all features of 

 organisms is constantly in progress ; that only a few of the 

 many varying individuals can survive ; that such variations 

 as happen to lie in a direction which fits the organism to its 

 environment will help that organism to survive in com- 

 petition with those having a less favorable direction ; that 

 the offspring of the surviving organism will inherit the 

 variation ; that some will vary in even higher degree ; and 

 that thus in time the variation can accumulate to a degree 

 which makes its possessor not only a new kind but better 

 adapted than its ancestors to those particular conditions. 

 Thus Nature acts to select certain characters, and the view 

 is known as NATURAL SELECTION. Translated into terms of 

 the chromosome mechanism, this means that the determiners 

 are not stable entities, but exist in a state of unstable equi- 

 librium such that they can produce characters in greater or 

 lesser degree' of intensity. As a matter of ract most of the 

 evidence we have accumulated upon this point seems op- 

 posed to the idea that the determiners are thus unstable, 

 and many investigators deny them all variability. More 

 recently, however, some apparently incontrovertible evidence 

 has been found which points to an inherent instability of 

 the determiners or unit characters, and their modifiability 

 by selection; and the Darwinian conception of evolution 

 by selection of such variations will probably prove correct 

 in the end. 



A modification of Darwin's explanation of the method of 

 evolution is that of De Vries, a Hollander still actively work- 

 ing. He maintains, on the basis of observational and ex- 



